The invention relates to flexible containers and to processes by which these containers are made. In a more particular sense, the invention relates to flexible containers and associated manufacturing processes having attributes well suited for use in the medical field.
Flexible containers for use in the medical field to process or store blood and tissue are well known. These containers are typically xe2x80x9clay flatxe2x80x9d bags, formed by heat sealing flat sheets of medical grade plastic material together, with access ports, about their peripheral edges. These bags are ideally suited for many uses in the medical field.
Still, the opposite sheets of flexible plastic that make up the walls of these bag are subject to non-uniform deformation during use. That is, the sheeting deforms non-uniformly depending upon the orientation of the bags and the location of materials within the bags, so that one area of the bag will have a different geometry than another. For example, the bottom of the bag, in which the materials naturally accumulate by gravity, will be wider than the top of the bag, which is normally free of materials. This disparity in the distribution of stress can lead to localized failures and leaks. Because of the non-uniform deformation, these bags are also not well suited for measuring precise fluid volumes.
It is possible to form bags having three-dimensional geometries using blow molding. However, blow molding requires the extrusion of parisons before the blow molding process. Furthermore, the porting of blow molded containers can pose problems.
It is also possible to form bags having three-dimensional geometries using vacuum molding techniques. However, these techniques required specialized vacuum molding tools and equipment and specialized heat sealing dies and procedures. Furthermore, vacuum forming can weaken a sheet of plastic by stretching the plastic material in nonuniform manner, creating areas where the film has been over-stretched and prone to failure when stressed.
The invention provides flexible, three-dimensional containers, which possess relatively precise, predefined geometries that are maintained during use. The invention also provides an assembly process by which flexible, three-dimensional containers can be made that meet exacting size and volume requirements, without the shortcomings of prior blow molded or vacuum formed containers.
One aspect of the invention provides a flexible container comprising first and second flexible sheets of plastic material having peripheral edges, which have been sealed together along a plane to create an interior chamber in the container. At least one of the first and second flexible sheets includes a preformed, stress-relieved region, which extends outside the plane and overlies at least a portion of the interior chamber.
In a preferred embodiment, containers that embody features of the invention are formed from first and second lay-flat sheets of plastic material, which can be softened by exposure to heat. The peripheral seal joins the two lay-flat sheets together along the plane, forming the interior chamber. The plastic material of the sheets has been extended or expanded into a preformed shape, which lies outside the plane, by the application of heat while positive pressure is applied to the interior chamber. The extension of the heat-softened material in response to positive pressure uniformly relieves material stress in the sheets. The combination of heat and positive pressure creates a robust container more resistant to stress-related material fatigue or failure than conventional lay-flat bags or bags having vacuum formed sides.
Other features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended claims.